John Cullum's Broadway career stretches 40 years, from Camelot to Urinetown, from the last golden roar of Lerner and Loewe to the first youthful yaps of Hollmann and Kotis, from Sir Dinadan-a noble addition to the medieval Table Round-to Caldwell B. Cladwell, a crafty port-a-potty monopolist in a many-eviled future where private toilets have been outlawed and a pay-per-pee program has been forced on the poor populace.

John Cullum's Broadway career stretches 40 years, from Camelot to Urinetown, from the last golden roar of Lerner and Loewe to the first youthful yaps of Hollmann and Kotis, from Sir Dinadan-a noble addition to the medieval Table Round-to Caldwell B. Cladwell, a crafty port-a-potty monopolist in a many-eviled future where private toilets have been outlawed and a pay-per-pee program has been forced on the poor populace.

It's a kind of progress, and his presence in this envelope-pushing musical spoof stills its troubled waters and makes it safe for us to proceed. His very billing is an all-clear signal.

If it still surprises you that Cullum would lend his name to that name, consider the role he turned down to do Cladwell-only the dream role of every seasoned actor: King Lear.

Right now he expected to be in San Francisco, carving up his kingdom among three conniving daughters. Instead, he's buck-and-winging it through a black and Brechtian satire installed at the Henry Miller Theatre after its big critical splash last spring Off-Broadway.

Well, maybe next Lear. He has already made five passes at the play. When he first came to New York, he stage-managed Sidney Walker's Lear-and got universally panned ("Almost every review said, 'The stage manager has a heavy hand on the thunder sheet'"). He was The King of France to Frank Silvera's Lear in a production Joe Papp staged up in Central Park. He directed a Lear in Seattle. And he was halfway through playing the title role in Scranton for director Paul Barry when he had to bow out because the financing for a TV-movie he had written (The Secret Life of Algernon) unexpectedly came through. This was the same Paul Barry he's now standing up in San Francisco and the same Paul Barry who gave him his first role in New York-Woody in a one performance-only of Finian's Rainbow. It was that role that got him an agent who kept putting him up for musicals while Cullum himself practiced his stagecraft in Bard after Bard.

The improbable combo of song-and-dance and Shakespeare proved quite combustible. "If I'd wanted to," Cullum says, "I could have made the decision then to stay in Shakespeare because I was a good Shakespearean actor and Joe Papp wanted to use me. The first season I was there, I understudied all the leads and got to go on as the chorus in Henry V when a guy sprained his ankle in the Broadway Show League. Alan Jay Lerner's assistant saw me doing that so, when I auditioned for Camelot, he said, 'That guy's a good Shakespearean actor.' They kept asking me back, and I couldn't figure why. They were looking for an understudy to Richard Burton, and that's how I got into musical comedy."

He got a little too good at doing Burton. "All the things I did sounded Burtonesque," Cullum admits in an uncanny imitation of Burton's rich, rolling Welsh. "It took me six years to get rid of this accent. Of course, I denied it for a long time, but, when I saw myself twisting my pinkie ring, I realized that's what Burton used to do, and that's when I said, 'I'm breaking this.'"

(He got in his Hamlet-in Milwaukee, in 1964-before he gave it up. And their strong friendship never faded. Burton beckoned Cullum aboard his last Broadway vehicle, a revival of Private Lives, with Kathryn Walker and a former Mrs. Burton.)

After Camelot, Cullum played a newspaperman to Robert Preston's Pancho Villa in We Take the Town. "I remember saying, 'If I had money, I'd put every cent into this show. There's no possible way this isn't going to make it. It has all the elements.'" Then tryouts began-and they loved it in New Haven. "Bob Preston always said, 'Those good reviews killed us,' because, from then on, the director and the producer talked to the writers through lawyers because they didn't want to change a thing. It closed in Philadelphia."

Lerner and Lane's On a Clear Day You Can See Forever required 13 auditions and the patience of Job to land. At one point during auditions, exasperated at reading that the very nonmusical Maximilian Schell got the show, Cullum took a film-the prolonged Hawaii. During one production standstill, he got an S.O.S. to come to Boston and replace the nonmusical star who had landed the lead, Louis Jourdan. "I learned the role in five days in a hotel room. No one knew I was there, except Barbara Harris and the production people. I had one rehearsal with Barbara on a Sunday and another rehearsal on Monday with the company, and opened that night." One last minute snag: "The stage manager wouldn't raise the curtain because I didn't have a contract." It took special dispensation from the union president, but, when that curtain did rise, Cullum became a Broadway star.

To give you an idea of his range, his Tonys were won for Shenandoah and On the Twentieth Century in roles played in movies by James Stewart and John Barrymore. "I have to say the role I have the greatest fondness for is Shenandoah-it would be stupid of me not to recognize that-but the greatest straight role I ever played was Hamlet. There's more satisfaction in playing it than any role you're ever going to play. I think probably the most fun role, because of the same kind of thing-you get about as much depth out of it as you bring to it-is La Mancha. That's a wonderful role."

Cullum is better known for "Northern Exposure" than Broadway exposure. Television, he says, "has given me a recognizablity I never had on Broadway and never will." The third episode of "Northern Exposure" brought this home to him. "There was a segment where one of the characters played three or four Broadway songs when the astronauts went up in space, and 'On a Clear Day' was the last one. And nobody knew that it was me singing. I thought, 'That's all right, let's just leave it that way. It's a different world.'"

And a denizen of Camelot and Urinetown would know about different worlds.